This invention generally relates to the transmission of information over telephone lines by using a telephone dial actuator to transfer information over a telephone from a sending telephone to a receiving telephone. These circuits have application in digital computer systems, call forwarding systems, remote dialing systems, paging systems, and credit card verification systems for example. In all these applications, the dial pulses must be counted accurately for reliable operation.
However, the path between a sending and receiving telephone may include many elements including myriad switching centers, amplifiers and other electrical circuits and electromechanical mechanisms. All these elements are sources of distortion and electrical noise. Atmospheric and other disturbances also can distort the signals and inject noise. All these factors tend to degrade the input signal and increase the difficulty in detecting dial pulses reliably.
In one prior dial pulse detector circuit, it is assumed that each dial pulse produces a transient pulse which a simple amplitude detector can discriminate as a dial pulse. A counter or other storage device stores information concerning the number of dial pulses which are received.
In another detection circuit, the high frequency content of signals on the telephone line is monitored to indicate the presence of tentative pulses with the characteristics of dial subpulses produced each time the dial actuator contacts open and close. An analog timing circuit establishes a timing window in response to a tentative pulse. The window is wider than the interval between successive dial subpulses but is narrower than the interval between corresponding dial subpulses of successive digits when a number greater than 1 is dialed.
The simple amplitude detection circuit is useful when both the sending and receiving telephones are connected through a single switching center. If several switching centers are intermediate the telephones, the amplitude of transients reduces and makes detection more difficult. The other detection circuit, which uses the analog timing circuits, is more reliable than the amplitude detection circuit. However, it is a rather complex circuit. Further, as all the components are fixed, it is difficult to alter the response or operation of this circuit, as may be desirable in some applications.
Therefore, it is an object of this invention to provide a pulse detection circuit for detecting pulses which conform to a known pattern in which the pattern can be altered easily.
Another object of this invention is to provide an improved circuit for detecting dial pulses produced on a telephone line.